Polybond Limited, Unit 6 William Street, Northam, Southampton, Hants. S014 5QH
Tel: 023 80 223266 Fax: 08700 527587
Q19 On Ferroconcrete Hulls
A Water Based Epoxy Paint System To Protect
In recent years, the development of Ferroconcrete boat construction has declined mainly due to the dictates of fashion and the apparent cheapness of the mass-produced GRP market. However, Ferroconcrete has never completely disappeared and remains an excellent, cheap, very strong alternative to steel and GRP when used for manufacturing ocean going cruising vessels and workboats etc.
In the area of finishing, much has been done over the years to improve the coating systems for concrete. In the early days of Ferroconcrete construction, builders and owners had to make do with a number of paint systems 'borrowed' from other industries, the resulting finishes being less than satisfactory. In some cases, this was another reason for putting people off going for Ferroconcrete as a boat building material. This has now changed with the availability of Polybond Q19 Water Based Epoxy System that has been especially adapted and improved for use on Ferroconcrete surfaces.
One of the main problems for builders of this type of hull has been finding a compatible, long term paint system, which will perform, satisfactorily in marine environments. Although some paint systems have been applied with some success - Chlorinated Rubbers and Epoxies both solvent and solvent-free types also Polyurethane's used as finishing coats, most tended to fail after a short time. The major problem being the moisture and concentration of salts which form in the skin at the time of application. One method used to try and overcome the problem was to acid etch prime. This process eats into the hard surface of the cured Ferroconcrete opening it and at the same time neutralising alkali salts that are present. This process does allow conventional solvent-based paints to penetrate but not sufficiently to ensure long term performance. In addition, the waiting period for drying can be lengthy. The dried film adhesion is therefore suspect and any moisture left in the concrete will tend to push the paint system quickly off the surface, particularly around the Boot-Top.
Should the paint film be broken at any time, particularly if the boat is in the water, a lateral breakdown will result as water moves under pressure across the hull surface. This is what can be described as 'sheeting' or severe delamination.
Adhesion to Wet Surfaces
The Polybond laboratories look into the problem of painting damp and wet porous surfaces, initially on building structures and then with Ferroconcrete hulls in sea water and camp up with some interesting results - it was obvious that to overcome the adhesion problem, you would need a water based product of some kind which when applied to 'green' damp surfaces offered excellent adhesion to the ferroconcrete despite the moisture and salts present. The coating would also ideally cut down on the normal protracted waiting times. Only one product proved to work up to 98% successfully and that was the modified Polybond Q19 System primarily developed to seal, waterproof and protect building surfaces above and below ground.
Waterproof and Vapourproof - It Controls Drying
Polybond Q19 Clear Sealer diluted with 10% fresh, cold water applied to Ferroconcrete in a two coat system will form a waterproof and vapour proof seal to a 'green' surface. It was also found that when applied immediately to a freshly plastered shell i.e within 24 to 48 hours, it allowed a certain amount of control over the curing process, perceptibly slowing it down, avoiding the need to 'water cure'. After painting, the deck framing can be covered over with polythene to inhibit evaporation inside the hull, dampening down should be continued inside every couple of days. Compressive strength of up to 597.38kg per cm² (8500lb per inch²) can be achieved using this method after a 28 day curing period. This eliminates stress cracking and the need to physically control drying on the external surfaces. Polybond Q19 helps to reduce the problems of accelerated drying and at the same time has advanced the painting process. Using the above method can cut Ferroconcrete drying times substantially even though the water based epoxy system takes a little longer than conventional coatings to dry out and cure. The factors controlling the drying are as follows :
The coating whenever possible should be applied in dry weather conditions and undercover. Polybond Q19 contains no solvents and can therefore be safely used in confined areas. The use of artificial drying aids is not recommended.
Cleaning and Removing Surface Salts
Surface salts can quickly, safely and effectively be removed from the surface of Ferroconcrete using an acid etch solution like Polybond Concrete Cleaner or similar mild acid based cleaning products. Once the solution has been safely washed off the surface, Q19 can be applied immediately. The use of strong Hydrochloric and other concentrated acids is not advisable. If they are used, the performance of the Q19 may be affected. Polybond Q19 diluted with 10% cold water covers approx 8m² per litre on surfaces of average porosity. In our experience, higher figures can be achieved and it is possible to get up to 10 to 12m² per litre when working on 'green' Ferroconcrete. Please refer to the table below :
|Green Concrete||24-48 hours old||10 to 12m² per litre per coat of Q19 Clear diluted with 10% cold water.|
|Semi Hydrated Hull||Over 5 days old||8m² per litre per coat.|
|Dry Concrete||Over 3 - 4 weeks old||6 to 8m² per litre per coat.|
|Q19 Clear Sealer||8m² per litre per coat.|
|Q19 Eggshell 7 Gloss||7.5m² per litre per coat.|
Another important aspect in the protective values of Polybond Q19 is found when 'skinning' existing boat hulls. The same drying problems occur when applying a Ferroconcrete skin on to metal or wooden hulls. The acceleration of drying can be worse in this instance as there is little or nowhere for moisture to go on the interface between the concrete and the surface to which it is applied. The Q19 can be applied in the same way to control the movement of moisture out of the skin and prevent stress cracking etc. It is important to remember two things however :
Relating to item one, the application of a second coat within 3 days can create an effective seal too quickly. This can be patchy and there is a possibility of a higher degree of moisture being trapped. Tan would be desirable at this stage. An even application of the Q19 is essential and spraying is normally employed to achieve this. As regards the second point, an over wet hull would almost certainly retard the Q19 cure and risk trapping excess moisture within the skin.
NOTE : Under no circumstances can a hull with a single coat of Q19 Primer be immersed in water until a full paint system has been applied - i.e. after a minimum of 28 days. Concrete can be immersed in water without protection but if this is done, the hull may suffer by absorbing salt or contaminated water and the damage could well be irreversible.
Methods of Finishing
Apply two coats of Q19 Clear Sealer diluted with 10% clean, cold water to the hull within 23 to 48 hours of plastering. After application of the initial coats, the controlled drying procedures should be continued on the inside of the hull as per normal building practise.
Keel Below Boot-Top
Once the hull is reasonably dry i.e 1-2 weeks depending on the weather and air temperature etc. an Epoxy primer (Polybond 'E' Primer) should be applied to the Q19 plus a minimum of 2 coats of Epiglo solvent based epoxy undercoat to prepare for the application of the anti fouling. All tin free and 'hard' types perform well on the Epiglo undercoat. These should be applied between 8 to 24 hours after the final Epiglo coats.
Surfaces can be finished using Q19 Colour Systems within 1 week of applying the second coat of Q19 Clear Sealer. This is to ensure that follow on coats key in satisfactorily. Apply a second and final coat of the Q19 colour (Spray recommended) within 24 hours of the first. If you decide to overcoat with other systems such as conventional Polyurethane Epoxy Gloss - i.e Epiglo of marine gloss or alkyd paints then use the Polybond 'E' Primer before overcoating and finishing. A minimum of 2 coats of Q19 Sealer will then be required. Allow 24 hours before overcoating especially in cold wet weather.
Proceed as for the keel. As an alternative to the standard finish an excellent long term non-slip surface can be obtained by casing Polybond Non-Slip Aggregate into the first coat of Q19 Colour. Allow this to dry overnight then brush off and apply a second coat of Q19 Colour to finish. To obtain a more flexible surface use a final coat of Polybond Chlorinated Rubber Paint as your finish coats. Please ask for details.
If the external hull surface is sealed with Q19, it is advisable not the paint the bilges until the concrete is completely dry and cured. Allow at least 3 to 4 weeks. The system is so waterproof that there is always a risk that of trapping moisture within the hull creating a 'sandwich' effect. Subsequent damage can cause corrosion of reinforcing wire. Lower areas nearer the keel can be treated when dry with our Epoxy Coal Tar finish to prevent bilge waste contaminating the concrete.
Bulkheads and Interiors
The Q19 system provides an excellent decorative finish for concrete bulkheads and boat interiors. Normally a 3 coats system is advisable on concrete as per Top Sides. The product is not recommended as a wood finish, please contact Polybond for any further advice.
Should the Q19 ever be damaged, repairs are easy to make - the surfaces should be rubbed down and washed thoroughly with fresh water and patch primed using several coats of Q19 Clear Sealer. The re-coat as for Top Sides. If the surfaces appear very smooth and dense then use our Polybond Concrete Cleaner prior to application.
Some curing aids used in Ferroconcrete construction can cause problems when painting. Care should be taken before using any artificial curing aid to determine its affect on paint systems. If in doubt always try a patch test and contact our technical department.
Apply Straight To 'Green' Concrete ...
Unit 6, William Street, Northam, Southampton, Hampshire, SO14 5QH
The claims made in this document are made in good faith and are based on exhaustive tests carried out over a long period.
Polybond Ltd., their agents and their distributors cannot accept any liability for the behaviour of their materials in individual cases.